Method of increasing thermoelectric power of pyrolyzed pyromellitonitrile/alcohol reaction product



July 28, 1964 J. E. KATON 3,142,644

METHOD OF INCREASING THERMOELECTRIC POWER OF PYROLYZED PYROMELLITONITRILE/ALCOHOL. REACTION PRODUCT Filed Dec. 22. 1960 THERMOELECTRIC POWER (av/g) 0 NO. L VACUUM "I N0. 2. A|R l No.3. vAcuuM2 No.4 AIR Z uo.s.- OXYGEN NQ6.- vAcuuMs I 1 l l i I I 5O 80 HO I40 I70 200 230 260 TEMPERATURE (C) INVENTOR. JOHN E. KATON ATTORNEY .alkyl is defined as 1 to 6 carbon atoms.

nit-ed tates atent 3,142,644 Patented July 28, 1964 fiice The invention relates to a new method of improving the thermoelectric properties of pyrolyzed pyromellitonitrile/alcohol reaction products by oxygen treatment at elevated temperatures.

In copending application Serial No. 11,897, filed February 29, 1960, pyrolyzed pyromellitonitrile/alcohol reaction products and the method of making them are described, and in copending application Serial No. 11,717, filed February 29, 1960, now US. 3,060,253, are described thermoelectric devices using as components pyrolyzed pyromellitonitrile/ alcohol reaction product bodies.

A new method has now been discovered for improving the thermoelectric power of P-type pyrolyzed pyromellitonitrile lower alkyl alcohol reaction products. Lower The reaction products result from the reaction of substantially 2 moles of alcohol with one mole of pyromellitonitrile. Pyrolysis is carried out in the presence of an inert atmosphere, e.g. in vacuum at temperature in the range of ZOO-700 C., preferably at temperatures above about 400 C. and more preferably above about 450 C. More details of the formation of the reaction products and the pyrolyzed reaction products are found in the two copending applications referred to hereinabove. This new method involved the oxygen treatment of these reaction products at elevated temperatures.

It is an object of this invention to provide a new and improved method for upgrading the thermoelectric power of P-type pyrolyzed pyromellitonitrile/alcohol reaction products.

These and other objects of the invention will become apparent as the detailed description of the invention proceeds.

The method of the invention involves treating P-type alcohol/pyromellitonitrile reaction products either in powdered or pellet form with oxygen, i.e. pure oxygen, and oxygen-containing gases such as air or ozone at elevated temperatures of about 50 C. to about 500 C., preferably about 75 C. to about 200 C. for a time suflicient to substantially increase the thermoelectric properties. Actually higher temperatures than 500 C. can be used but treatment time at these higher temperatures must be very short in order to avoid completely carbonizing the material and destroying its thermoelectric propertiesthe higher the temperature of treatment the shorter the time necessary to obtain a certain increased thermoelectric power. Times of treatment will vary from about 10 minutes to 12 hours at high temperature of the order of 500 C. or higher and from about 1 day to a month or more at low temperatures of the order of 100 C. or lower. Not as long a time of treatment is required with oxygen or ozone as compared to treatment with air to acquire the same amount of increased thermoelectric power. -The oxygen for treatment of the P-type alcohol/pyromellitonitrile reaction products can even be provided by materials which decompose to give-off oxygen, e.g. by mixing or treating the reaction products with a peroxide such as benzoyl peroxide. In any event the reaction products are heated at elevated temperatures for a time suflicient to substantially increase the thermoelectric properties of the products.

The invention will be more clearly understood from the following detailed description of specific examples thereof read in conjunction with the accompanying drawing which is a graph showing a family of curves representing various types of treatment of a pellet of pyrolyzed methanol/pyromellitonitrile reaction product with temperatures of treatment being plotted as abscissas and thermoelectric powers as ordinates.

Example 1 ice bath and filtered to recover a light greenish-yellow solid. This solid product weighing 17.35 g. does not melt but starts to change color at -155" C., turns blackgreenish at -165 C. and by 200 C. has turned purplish-black. These observations of color change on heating material were carried out on only a small sample of the material. A sample of 16.2 g. of the 17.35 g. of material was subjected to sublimation treatment at 265 C./0.80.2 mm. of Hg absolute pressure for 18 hours. The weight of non-sublimable residue material recovered after sublimation treatment was 12.3 g. A sample of this non-sublirnable residue was pressed in a /2" diameter die using 20,000 pounds force to form a pellet having the thickness of about 0.325". This pellet was then heat treated as follows: 300" C. for 24 hours, 400 C. for 24 hours, 500 C. for 24 hours and 600 C. for 24 hours all at 0.1-0.2 mm. of Hg absolute pressure. The weight of the pellet before heat treatment was 1.6317 g. and after heat treatment was 1.2455 g., i.e. a weight loss of 23.6% as a result of heat treatment.

Example 2 V .AT of 83 C. The determined thermoelectric power was +22 microvolts/ C. The pellet was then mounted in a cell where the atmosphere surrounding the pellet could be controlled and the thermoelectric measurements could be made by copper/constantin thermocouples.

The first series of measurements at various temperatures were made with the sample under high vacuum of the order of about 0.5 mm. of Hg. In this series of runs the bottom side of the sample was heated and the thermocouples measured the differential temperature for thermoelectric power determinations. The temperature shown below in the tables is the average temperature of the two thermocouple readings. The thermoelectric power generated was measured by connecting the copper leads of the two thermocouples to a potentiometer and balancing the voltage obtained with a standard cell and a calibrated resistor, giving sufficient information to calculate the thermoelectric power of the sample. In the table below are shown the number of hours the pellets were maintained at a certain temperature including heating up time and the thermoelectric power of the pellet determined at the end of this period of time. The following is a summary of the data from this vacuum run.

third run under vacuum was made and this run is sum- Time, hm Tempew TEPYI marized in the table below.

ture, 0. pV./ C.

Time, hrs. Tempera- TEP, 18% 92 6 +8. 7 5 ture, C. v./ C. a? tit-2 12'; g5- gg-g g-g 13% 23813 8 9 48% 126. 7 +21 194. 6 53.. 143.1 +28. 5 77 156.2 +23.s 10 96. 176.5 +19.9 1 TEP means thermoelectric power. 0 6 In the data above it is seen that some samples are label- The Six runs described above are summarized in a ed F e h some mlhus thermoelectric Power- Thls 1s semi-quantitative fashion in the figure wherein temperaah mdleatlohes to Whether the e h e has P'tYPe or tures are plotted as abscissas and thermoelectric power type conduetiv ty with the plus lndicatingthe P-type q as ordinates to give the six curves representing the six the hhhus mdleatlhg h The tunes Shown 1h runs. It is readily apparent from an examination of the hours are cumulatlve, 1.e. 18 /2 hours were used to heat curves that oxygen treatment of pyrO1yZedmethane1/pyro the semhle uP to and hold It at 92-60 betore the ther mellitonitrile reaction products f P-type conductivity inmoeteetne Power measurement Then t temperature creases the thermoelectric pow thereof when the treat- PP to the bottom of the Pellet e lhereased and at 20 ment is carried out at elevated temperatures. The curves the end of 4 /2 hours the thermoelectric power was measf the figure are plotted f the data of Example 2 and ured at 113-90 e As was Prevlhusly Stated the represent in the same order as the curves are numbered perature reported is the average of the two thermocouple the Six runs beginning With the first vacuum rum Teahhgs- An alternative method of making the improved oxygen- At the end of the Vacuum after the Sample had treated pyrolyzed pyromellitonitrile/lower alkyl alcohol e allowed to cool h Sample F was Opened to the reaction products is to pyrolyze the products in the presair. Then another ser es of heating the pellet from the ehee ofexygem In Such an alternative process much bottom in steps for various length of time began with air Shorter pyrolysis times should he used at high temherw surrounding the sample. The results of this series of tests three to avoid completely earhohizihg the material and is reported in the table below. The measurements were 0 destroying its thermoelectric propertiee carried out 1n the same manner as were the measurements The oxygemtreated pyrolyzed llit it fl /l for the vacuum run above The data are as follows: alkyl alcohol reaction products of this invention can be employed as the pyrolyzed pyromellitonitrile/ alcohol re- Tlmehrs at??? 33 action product bodies in the thermoelectric devices disclosed in copending application Serial No. 11,717, filed 74,6 February 29, 1960, the disclosure of which is hereby in- 3 3 corporated herein by reference. i 1 Although the invention has been described in terms of specified embodiments which are set forth in considerable After this first run in air a second run was made in detail, it should be understood that this is by way of ilvacuum and the results of this vacuum run are reported lustration only and that the invention is not necessarily in the following table. limited thereto since alternative embodiments and operat ing techniques will become apparent to those skilled in Time, g f h the art in view of the disclosure. Accordingly modifications are contemplated which can be made without deloao +21 0 parting from the spirit of the described invention. 141,1 +19: 7 What is claimed is:

g3? l. A method for improving the thermoelectric proper- 71% 22314 +211 ties of P-type pyrolyzed reaction product, said pyrolyzed 74% 249-4 reaction product being made by the process comprising:

. (1) heating at at least reflux temperature a mixture of Next a seqond m e matte l measurements pyromellitonitrile and a lower alkyl alcohol to form a reand calculations bemg .made m slmilar manner to the action product having substantially 2 moles of alcohol other runs. The data 1s summarized in the table below. Per mole of pymmenitonitrhe, (2) separating the reac Time hm Temper? TEP 1 tion product of step 1 from the reaction mixture, and (3) ture, C. ,tv./ b. heating in an inert atmosphere the reaction product sep arated in step 2 at a temperature in the range of about 4 87.5 +23.2 ZOO-700 C. for a time suflicient to produce the desired 5 15%? thermoelectric properties, comprising heating said pyrolyzed reaction products prior to use in a thermoelectric After completing the second run in air instead of makdevice in the pfesenfle of free Oxygen at suificiently ing another vacuum run, a run was made with the sam- Vated P III h range P about 50 9 abqut ple surrounded by an atmosphere of oxygen. A slight 500 C. for a period of time sufficient to substantially inpositive pressure was maintained in the sample cell feed- Crease the thermoelectfl? Power of Py y l ing i the l f an Oxygen li d Th d t f 2. The method of claim 1 wherein said alcohol is meththe oxygen run is shown in the following table. M101- 3. The method of claim 1 wherein the reaction product Time, hrs. Temp ra- TE P, separated in step 2 is formed into a pellet under pressure 0 prior to step 3.

7 O 4. The method of claim 3 wherein said alcohol is methp, 66 +3518 a n 01 zo s7 +35.0 24% 117 +367 gi References Cited in the file of this patent Berkmann: The Chemistry of Acetylene and Related To complete the series of runs after the oxygen run a Compounds (1948), page 80. 

1. A METHOD FOR IMPROVING THE THERMOELECTRIC PROPERTIES OF P-TYPE PYROLYZED REACTION PRODUCT, SAID PYROLYZED REACTION PRODUCT BEING MADE BY THE PROCESS COMPRISING: (1) HEATING AT AT LEAST REFLUX TEMPERATURE A MIXTURE OF PYRMELLITONITRILE AND A LOWER ALKYL ALCOHOL TO FORM A REACTION PRODUCT HAVING SUBSTANTIALLY 2 MOLES OF ALCOHOL PER MOLE OF PYROMELLITONTRILE, (2) SEPARATING THE REACTION PRODUCT OF STEP 1 FROM THE REACTION PRODUCT SEPARATED IN STEP 2 AT A TEMPERATURE IN THE RANGE OF ABOUT 200-700*C. FOR A TIME SUFFICIENT TO PRODUCT THE DESIRED THERMOELECTRIC PROPERTIES, COMPRISING HEATING SAID PYROLYZED REACTION PRODUCTS PRIOR TO USE IN A THERMOELECTRIC DEVICE IN THE PRESENCE OF FREE OXYGEN AT SUFFICIENTLY ELEVATED TEMPERATURES IN THE RANGE OF ABOUT 50 TO ABOUT 500*C. FOR A PERIOD OF TIME SUFICIENT TO SUBSTANTIALLY INCREASE THE THERMOELECTRIC POWER OF SAID PYROLYZED PRODUCT. 